The present invention relates to nondestructive testing and more specifically to digital bond testing of composite aircraft structures to sense the presence of manufacturing faults or damage.
Prior art patents utilize some form of ultrasonic probing imaging to detect flaws in parts or members. Several analyze the part at multiple frequencies for maximum return (U.S, Pat. Nos. 5,309,765; 4,170,144; 5,417,218; 4,169,385). Several patents also use a gate to isolate the response at a particular depth or location of the part (U.S. Pat. Nos. 4,167,879; 4,170,144). In contrast, the present invention performs a calibration operation by measuring the maximum difference in the waveform responses between standard (undamaged) and flawed portions of the test article. The gating operation determines the maximum variation between the reference waveform and the test waveform for multiple positions on the part resulting in improved sensitivity.
Query: Is gating a comparison? No, this terminology arises from the manner in which the waveforms are digitized. When a trigger is generated by the motion control electronics of a scanner, the digitizer takes a series of samples of the waveform at the rate of 1 million samples per second. If 500 samples are to be taken, the signal waveform is sampled over a period of 500 microseconds, and these voltage values are used to generate the analog representations of the waveforms. A gate refers to a specific delay after the initiating rigger. Gate 250 would be the time interval 250 microseconds after the trigger signal. Since that delay yielded the largest difference signal between the flaw and reference waveforms at 20 kHz. The sample at that delay would yield the largest response to that type of flaw during a scan.